Welch, William Henry (1850-1934), American pathologist and bacteriologist. Welch played a major role in the introduction of modern medical practice and education in the United States. As dean of the Johns Hopkins Medical School he demanded of his students a rigorous study of the physical sciences and an active involvement in clinical duties and laboratory work. Under his direction Johns Hopkins became a model for American medical schools. In his own investigations he is remembered for his demonstration of the pathological effects of diphtheria toxin and for his discovery in 1892 of the bacterium of the genus Clostridium (C. perfringens) that causes gas gangrene.